1. Technical Field
This invention relates to a process for producing highly water-absorbing resins improved in absorbency for water and body fluids and furthermore improved in strength in the state of a swollen gel.
2. Prior Art
Known resins capable of absorbing water in large amounts include such highly water-absorbing resins as partially hydrolyzed starch-acrylonitrile graft polymers, partially neutralized polyacrylic acid species, polyethylene oxide species, partially hydrolyzed polyacrylonitrile species and polyvinyl alcohol species. Among them, partially neutralized polyacrylic acid species are particularly useful.
These highly water-absorbing resins are used in various fields, for example as body fluid absorbents, which absorb body fluids and prevent them from leaking, in sanitary products such as sanitary napkins, tampons, diapers and so on, and further as seed coverings, water leak stoppers, thickening agents, dew formation inhibitors, sludge coagulants, driers, moisture conditioners, etc.
Such highly water-absorbing resins should meet the following requirements: that when they are brought into contact with water or body fluids, the phenomenon of lump formation should not take place, that they should have great absorbencies for water and body fluids with high rates of absorption, and that the gels formed upon absorption of water or a body fluid by them should have great strength. Even partially neutralized polyacrylic acid, which is one of preferred highly water-absorbing resins, cannot always meet these requirements.
Therefore, various attempts have been made to improve the absorbency and gel strength of partially neutralized polyacrylic acid by carrying out crosslinking with a crosslinking agent such as ethylene glycol diglycidyl ether in the step of polymerization or after polymerization. Such attempts are listed below.
Japanese Kokai Tokkyo Koho No. 44627/1982: Said polymer is dispersed in a mixed solvent composed of water and a hydrophilic organic solvent, with a crosslinking agent having two or more functional groups, and crosslinking is carried out.
Japanese Kokai Tokkyo Koho No. 42602/1983: Said polymer is dispersed in a dispersion medium and brought into contact with a crosslinking agent, whereby the polymer is crosslinked on the grain surface thereof.
Japanese Kokai Tokkyo Koho No. 117222/1983: Said polymer is crosslinked with a crosslinking agent having two or more functional groups in an inert solvent in the presence of water.
Japanese Kokai Tokkyo Koho No. 62665/1984: Said polymer is crosslinked in a hydrated form with a crosslinking agent having two or more functional groups.
Japanese Kokai Tokkyo Koho No. 147475/1985: Said polymer is crosslinked with a crosslinking agent having two or more functional groups in a hydrophilic inert solvent in the presence of water and a mineral powder.
Japanese Kokai Tokkyo Koho No. 177004/1985: Said polymer is crosslinked with a crosslinking agent in an inert solvent in the presence of a hydrophilic polyhydric alcohol and a mineral powder.
Japanese Kokai Tokkyo Koho No. 186506/1985: In subjecting a dispersion or a suspension of an aqueous monomer solution in a hydrocarbon or halogenated aromatic hydrocarbon to polymerization, an oil-soluble cellulose ester or cellulose ether is used. After polymerization, the moisture content is adjusted and crosslinking is carried out using a crosslinking agent having two or more functional groups.
Japanese Kokai Tokkyo Koho No. 199010/1985: The monomer is polymerized in the presence of a specific polycationic epoxy compound (a reaction product of an imidazole and a halomethyloxirane compound).
Japanese Kokai Tokkyo Koho No. 255814/1985: A mixture of the polymer and a mineral powder is sprayed, with stirring, with a crosslinking agent and water, the crosslinking reaction is effected by heating and, thereafter, water is distilled off.
Japanese Kokai Tokkyo Koho No. 69812/1986: During or after polymerization of the monomer, a monoglycidyl compound is subjected to reaction.